This invention relates to a method and apparatus for reducing the density and increasing the bulk of a formed fibrous web, and more particularly to directing a gaseous medium, such as air, onto the surface of a fibrous web so as to decompact the fibrous web throughout the entire thickness of the web.
One well known prior art approach for obtaining a uniform, low density, high bulk fibrous web is to dry form the web from a gaseous suspension such as air. Such webs are formed by conveying the fibers in an air stream and depositing them in a randomly arranged and intermingled fashion on a foraminous surface in the form of a fibrous web or batt. The speed of web formation in air is slow when compared to the speed of papermaking machines that form a web from a water suspension at a rate in excess of 4,000 feet per minute. It would, therefore, be highly desirable to be able to modify the structure of a web formed web either at an off line station or at some point in a conventional papermaking process in order to achieve the low density, high bulk characteristics normally attributed to a web made by an air forming process.
U.S. Pat. No. 3,490,103-Asaka, et al. discloses that the structure of a pre-formed pulp sheet can be modified so that both surfaces are highly decompacted while, as indicated in FIG. 3, the central core region remains relatively dense. In Asaka, et al., as mentioned at column 1, lines 25 and 36 and again at column 2, line 12 a moistened pulp sheet is placed in tight engagement between a pair of wire mesh screens and then air is directed under pressure against the surface of the sheet in order to decompact the fibers in the surface of the sheet.
U.S. Pat. No. 3,556,931-Champaigne is concerned with modifying the structure of a wet formed pulp board sheet by decompacting both surfaces of the sheet without substantially affecting the density of the central region of the pulp board sheet. Champaigne accomplishes this by treating the surfaces of the pulp board sheet with a debonding agent to weaken the bonds between fibers in the surfaces and then mechanically working, or flexing, the sheet to further break bonds in the surfaces of the sheet, then, as stated at column 2, line 14 tightly sandwiching the pulp board sheet between foraminous members, and directing air blasts at the surface of the sheet to fluff the relatively unbonded fibers.
Another disadvantage of the prior decompaction art, as represented by Asaka et al. and Champaigne, is that those patents require the fibrous sheets to be moistened before the surfaces can be decompacted. It has been found that when the initial web has been treated or formed to have a basis weight in the range of 20 to 60 pounds per ream and a breaking length of between 50 to 500 meters, the web need not be premoistened in order to achieve substantial decompaction throughout the entire thickness of the web.
In describing the invention, it is convenient to use the parameter, non-directional breaking length, to indicate the initial strength of the interfiber bonds within the web. As is well known in the papermaking art, the breaking length of a web is the length of a strip of the web required to cause the strip to break under its own weight. Furthermore, since it is quite common for the breaking length of a strip of a web that runs in the machine direction to differ from the breaking length of a strip of the web that runs along the cross machine direction, for the purposes of this application, the term non-directional breaking length is defined to be the square root of the product of the machine direction and cross machine direction breaking lengths.
It is therefore, one object of this invention to provide an improved method and apparatus for producing a low density, high bulk fibrous web.
It is another object of this invention to substantially reduce the density and increase the bulk of a web formed fibrous web, having certain prescribed characteristics.
Another object of this invention is to substantially reduce the density and increase the bulk of a wet formed fibrous web throughout all regions of the web.
And yet another object of this invention is to treat a wet formed fibrous web so that it will have the density and bulk characteristics of an air-formed fibrous web.